High energy density, rechargeable electrochemical cells are known in the art. A rechargeable cell is theoretically capable of charging and discharging indefinitely. In producing a rechargeable battery system, a material for the cathode is selected. In certain instances, the cathode material is in the form of a liquid which allows reactions to readily take place. However, when in the form of a liquid, provisions are made to keep the cathode active material away from the anode, otherwise self-discharge can occur. As an alternative, the cathode material can be in the form of a solid which is essentially insoluble in the electrolyte. The solid cathode material is selected such that it absorbs and desorbs the anode ion because solubility of the anode ion occurs reversibly during operation of the cell. Such a solid cathode can be capable of intercalation of ions which are solubilized by the electrolyte. The electrolyte is selected to permit electroplating of solubilized ions at the anode. The plating of ions at the anode occurs during recharge of the cell and the intercalation of the cathode occurs during discharge of the cell.
Chevrel-phase compounds (CPs), also referred to as Chevrel materials, include an invariant portion which may consist essentially of molybdenum and a chalcogen. The chalcogen can be selected from elements in Group 16 of the Periodic Table, including sulfur, selenium, tellurium or mixtures of these, with or without minor amounts of oxygen. Ordinarily, this fixed portion has a stoichiometric formula of Mo6Zy where Z represents the chalcogen and y is usually between about 7.5 and 8.5, most typically about 8. The unique crystal structure of the materials permits intercalation of metals, so that the overall stoichiometry of the Chevrel-phase material can be represented as MxMo6Zy where M represents the intercalated metal and x may vary from 0 (no intercalated metal) to an upper limit which may be about 4 or less depending upon the particular metal.
Ternary CPs are a unique class of cluster compounds which exhibit remarkable magnetic, thermoelectric, catalytic, and superconductive properties. The crystal structure of CPs consists of Mo6-octahedron clusters surrounded by eight chalcogen (e.g., S or Se) atoms at the corners of a distorted cube. For example, Mo6S8 units are linked with each other and form a three-dimensional framework with open cavities/channels that can be filled with wide-variety of guest atoms and form ternary CPs MxMo6S8 (0<x<4). However, Mo6S8 binary CPs cannot be synthesized directly and indirectly stabilized via leaching metal from their ternary counterparts. Mo6Se8 binary CPs include an iso-structure wherein Mo6Se8 clusters are rotated approximately 26° about the body diagonal (3 axis) of the rhombohedral symmetry (R3) which allows for bonding of Se atoms of one cluster to a Mo atom of a neighboring unit. The resultant three-dimensional Mo6Se8 framework has open cavities/sites that can be filled completely in the MxMo6Se8 CPs into triclinic (P1) forms due to intrinsic lattice instabilities. Among the three different families of CPs (Mo6Z8, Z═S, Se, Te), sulfide CPs have high ionic mobility at room temperature which allows them to transport monovalent (Li+, Na+) and bivalent (Mg2+) cations, and to act as a cathode for rechargeable batteries.
Energy is released upon intercalation of the metal into the CPs and as the intercalation process is partially or wholly reversible with certain metals, the CPs can be used as cathodes in electrochemical cells.
A cell with a lithium anode and a Chevrel-phase cathode of the formula LixMo6S8 can be subjected to a charge cycle in which lithium is removed from the Chevrel-phase by the applied electrical energy. In a discharge cycle, the lithium is re-intercalated into the Chevrel-phase releasing energy as electrical energy. The reaction mixture containing lithium, molybdenum and sulfur for direct formation of the lithium-intercalated CPs can be prepared by heating a precursor mixture. The precursor mixture including a heat-liable lithium compound together with molybdenum and sulfur, typically as a mixture of MoS2 and free Mo. Upon heating, the heat-labile compound yields volatile decomposition products which may be swept from the mixture, e.g., by a stream of inert gas, leaving behind the lithium, molybdenum and sulfur to form the Chevrel-phase material.
Intercalation reactions in typical battery development have focused on the use of alkali metals, specifically lithium as anodes. In comparison, there has been less research with respect to the use of alkaline earth metals, such as magnesium, for use as anodes and the use of cathodes capable of intercalation of alkaline earth metal ions.
It is known in the art to use lithium ion batteries for a wide variety of energy storage applications due to their very high energy density and flexible design. In considering alternative materials to lithium in producing electrochemical batteries, it is acknowledged that magnesium-based energy storage systems may be considered suitable alternatives because magnesium is environmentally safe, cost effective and abundant in the earth's crest. Further, magnesium is bivalent and theoretically capable of rendering higher volumetric capacity than lithium.
It has been found that conventional salts like Mg(ClO4)2, Mg(CF3SO3)2, Mg[(CF3SO2)2N]2 and the like, in various non-aqueous solvents develop surface passivation on a magnesium anode and effectively block Mg2+ transport. Relatively fast and easy intercalation of Mg2+ ions at room temperature makes CPs a preferred material of the cathode for magnesium batteries. However, synthesis of thermodynamically unstable Mo6S8 is challenging. Typically, CuCP(CuxMo6S8) is synthesized by solid state reactions of elemental blends of copper, molybdenum, and MoS2 powders in an evacuated quartz ampoules at a temperature of approximately 1150° C. for one week or by a molten salt approach including heat treatment at approximately 850° C. for about 60 hours under an argon atmosphere. Both of these approaches require chemical leaching in solution for several days at room temperature for complete removal of the copper.
The conventional methods are not convenient approaches for large scale manufacturing process. For example, an elemental blend sealed in evacuated quartz ampoule and heated at high temperature (˜1273 K) for a long duration (˜7 days) to obtain CuxMo6S8 results in high manufacturing costs. Further, a significant disadvantage is the high vapor pressure of sulfur inside the ampoule during heating which causes a safety hazard. Furthermore, the final product obtained from the quartz ampoule has non-stoichiometry and excess sulfur that may be required to compensate for sulfur vapor loss during heating. Attempts to use metal sulfide (CuS, MoS2) instead of elemental sulfur to avoid the high vapor pressure, has resulted in a synthesis time which is unreasonably long for amenable large-scale production. Moreover, it was found that there can be difficulty in forming a reaction product at high temperature, e.g., the Chevrel phase may not be obtained and instead a sulfur deficient phase can form.
In an attempt to reduce total synthesis duration during the solid-state reaction method, cold-pressing and hot pressing have been employed as alternative means for the synthesis of a CuxMo6S8 phase. Neither the milling time nor the sintering temperature has been optimized for the synthesis of CuxMo6S8. However, it was found that pressure-assisted sintering at elevated temperature ˜1123-1473 K was capable of reducing the synthesis duration to ˜5-8 hours as compared to 7 days required for solid-state synthesis in an evacuated quartz ampoule.
CPs also have been synthesized from soluble sulfide precursor, e.g., polythiomolybdate and metal salt, which form a chelated complex in ether or methanol solvent and directly forms the desired Cu2Mo6S8 phase when heated at ˜1073-1273 K under hydrogen atmosphere. An alternative soluble precursor method was proposed for the synthesis of nickel and lithium ternary CPs.
Obtaining Mo6S8 CPs from known soluble sulfide precursor methods requires the use of hydrogen gas during sulfurization. Final reduction of sulfur compounds to desired Chevrel phase at elevated temperature needs strict regulation and skills, and poses a safety concern for the synthesis of CPs.
Thus, there is a need for improvements in electrochemical cells incorporating Chevrel-phase cathodes and in the synthesis methods for the CPs, and furthermore in the use of alkaline earth metals, such as magnesium, as anodes. Moreover, there is a need in the art to develop time-saving approaches and methods for the direct synthesis of CPs. In this respect, high energy mechanical milling (HEMM) may be employed as a scalable, time-saving approach for direct synthesis of ternary metal CPs (MxMo6Z8; Z═S, Se) using MZ, Mo, and MoZ2 as the precursor. The invention provides easy, rapid and facile precursor approaches for the synthesis of CPs for use as a cathode and magnesium-containing material for use as an anode in an electrochemical cell.